Flow injection analysis with intermittent flow

ABSTRACT

Continuous flow analysis with an unobstructed, unsegmented carrier stream transporting a sample plug through a main conduit from a sample injection station to a flow-through detector. One or more reagents are injected in the sample plug on its way through the main conduit, the dispersion being controlled to optimize the conditions of chemical reaction. The reagents are added by being pumped in intermittently in merging streams, at least one of the solution delivering devices being activated and deactivated at pre-programmed time intervals during each sampling cycle. Each cycle is initiated by injecting the sample solution at the sample injection station. The solution delivering devices can have various pumping rates and at least one pump may be an automatic burette. The delivery is controlled by a timer or microcomputer, and the transport of solution can be momentarily stopped before the sample plug enters or while it is inside the flow-through detector.

Flow injection analysis, FIA, has opened up new areas within the field of analysis. FIA is a continuous analysis system in which discrete volumes of sample solution are successively injected into a continuous, unobstructed carrier stream. The sample solutions react with the carrier stream and a detector for registering the results of the reactions is placed downstream from the point of injection. The principles for FIA are described in detail in our U.S. Pat. No. 4,022,575.

One of the most important developments of the FIA theory (see our U.S. Patent Application Ser. No. 48,002, filed June 13, 1979, Stop-Flow) increases the residence time by decreasing the velocity of the carrier stream instead of increasing the length of the reaction coil, because lengthening of the sample zone path leads to increased dispersion, while decrease of the pumping rate results in lesser spreading of the sample zone. Should, however, the carrier stream cease to move, the dispersion of the sample zone will stop (except for a negligible contribution due to molecular diffusion) and the dispersion, D_(t), will become independent of time. Thus by applying intermittent pumping, one can gain reaction time during the stop interval when the carrier stream does not move. If the sample zone is stopped within the flow cell itself, it is possible to record the change of, say, optical density, caused by the reaction between the sample component and the reagent in the carrier stream. The obvious prerequisite for such a reaction rate measurement to be successful is that the movement of the carrier stream be able to be exactly controlled from the operational pumping rate used to complete standstill, and that always the same section of the sample zone be able to be reproducibly held within the flow cell for measurement. In practice, this is best achieved by using an electronic timer which is activated by a microswitch connected to the injection valve. Thus any delay time as well as any length of stop time can be chosen so that it suits the reaction rate of the particular chemistry.

The invention will now be described in more detail with reference to the accompanying drawings, of which

FIG. 1 is a comparison between continuous FIA and stop-flow,

FIG. 2 is a modified stop-flow system,

FIG. 3 is a graph concerning the system according to FIG. 2.

FIG. 4 is the intermittent system according to the invention,

FIG. 5 is a graph illustrating the system according to FIG. 4,

FIG. 6 shows merging zones,

FIG. 7 shows merging zones with intermittent pumping,

FIG. 8 illustrates the system according to FIG. 7, and

FIG. 9 shows the intermittent addition of two or more reagents to a sample.

FIG. 1 demonstrates the principles of the Stop-Flow FIA method. A dyed sample zone is injected into a colourless carrier stream and the absorbance is recorded by means of a colorimetric flow-through cell. All curves are recorded from the same point (S) by injecting 26 microliters of the same dye. Curve a shows continuous pumping, curve b 9 seconds of pumping, 14 seconds stop and continuous pumping again, and curve c, the dotted line, indicates the curve which would have been registered if any chemical reaction had taken place within the flow cell during the 14 second stop interval.

Reaction speed measurements are often used within clinical chemistry and therefore the enzymatic determination of glucose based on the use of glucose hydrogenase coupled to the spectrophotometric measurement of the coenzyme AADH became the first use of the Stop-Flow FIA system.

Another interesting example is the determination of SO₂ in wine, based on the well-known West-Gaeke method in which a pink-coloured compound, formed by the reaction between pararosaniline and sulphur dioxide and catalyzed by formaldehyde, is measured at 580 nm. While the determination of sulphur dioxide (which is always present in wine as a preservative) can be performed on samples of white wine by a simple direct measurement; the colour of red wines interferes, especially because for different coloured wines it presents a variable blank. This, however, can be corrected in each individual sample by measuring the increase of absorbance due to the reaction between sulphur dioxide and pararosaniline while the sample zone is kept still within the flow-through cell (cf. FIG. 1).

Since the premixed reagent is not stable and changes with time, the simplest stop-flow system had to be slightly modified (FIG. 2).

The sample (10 μl) is injected into a carrier solution of pararosaniline (0.08% in 0.3 M H₂ SO₄) to which is added a solution of formaldehyde (0.5% in 0.3 M H₂ SO₄) which catalyses the colour-forming reaction. After mixing of the reagents in the 15 cm (0.5 mm I.D.) coil, the sample zone is, by means of the electronic timer, stopped in the flow-through cell of the spectrophotometer. This testing arrangement permitted the determination of sulphur dioxide in wines at a rate of 105 samples per hour and with the analytical readout available 23 seconds after sample injection.

FIG. 3 shows the calibration record for sulphur dioxide using the stop-flow system in FIG. 2. The concentrations are given in ppm SO₂. The delay time was chosen slightly longer than the residence time, i.e., the samples were stopped shortly after the peak maxima had been passed. As the stop time was identical in all cases (15 sec.), the analytical result is given as the peak increase during the stop interval. To the right are shown the records for the determination of the free sulphur dioxide contents in two white wines, Touraine Blanc, 29 ppm; and Gumpoldskirschner, 1977, 18 ppm.

It is interesting to note that stop-flow injection analysis can be performed on the various sections of the dispersed sample zone i.e., not only by monitoring that part corresponding to the peak maximum, but by stopping in the flow cell any segment on the tailing portion. This is described in detail in our U.S. Patent Application Ser. No. 48,002, filed June 13, 1979, Stop-Flow.

Thus, in contrast to conventional stop-flow systems in which sample and reagent solutions are completely mixed by force in a special chamber, the stop-flow injection method readily allows adjustment to obtain optimum measurement conditions. Also, since the mixing takes place in a simple tubing without special auxiliaries, the apparatus is simple to construct, and since during the stop period no reagent is being pumped, the stop-flow method is less demanding on reagent consumption than the continuous flow approach. It should, however, be pointed out that if the rate of the sample zone dispersion were slower than the rate of the chemical reaction to be measured, the result would be distorted and therefore FIA is not suitable for extremely fast reaction rate measurement.

FIG. 4 shows a flow injection system with intermittent pumping with two pumps in such a way that they operate at different time intervals in a prefixed time sequence. Within each separate sampling cycle at least two pre-programmed changes of velocity of the carrier stream take place. The simplest use of such a system is to increase the sample frequency by increasing the wash-out speed of a coil and a flow cell by stopping the first pump and starting the second one immediately after the top of the peak has been measured. This is simply achieved by using a FIA system such as that depicted in FIG. 4, where the first pump of slower pumping rate delivers the reagent stream while the second pump, having higher pumping rate, delivers only wash solution (water).

The sample (S) is injected into the reagent stream (R) propelled by pump I. After the peak maximum has been registered by the detector, pump I is stopped and pump II activated and used to wash the sample out of the system (pumping rate y>x). After a preset time pump I is restarted and pump II stopped, thus permitting a new sample injection. All lines are 0.5 mm I.D., the tube length from the point of injection to the T-joint is 25 cm, and from there to the flow cell (including the coil) 75 cm. This approach allows an increase of the sampling frequency and greatly reduces the consumption of reagent solution.

FIG. 5 shows a record (A) obtained with the FIA system in FIG. 4 operated with intermittent pumping by injecting 20 μl of a coloured dye solution. Pumping rate x=0.9 ml/min; y=4.0 ml/min. Since the residence time of the sample in the system was 18s, pump I was preset with a delay time of 19.9s during which it was operated, while pump II was stopped. After the delay time, pump I was stopped and pump II operated for a preset period of 12 secs. and the sample flushed out of the system, whereupon pump I was reactivated and pump II stopped. As seen on the figure, this operation allowed the sampling rate to be approximately doubled compared to continuous pumping of pump I alone (B). The disadvantage of merging zones by multiple injection with a valve is that one has to have a reagent supply circuit where the expensive solution is held in reservoirs and diluted by successive addition of the carrier stream. This is not the case in intermittent pumping, which is also technically more feasible.

The high reagent consumption is the main disadvantage of all continuous flow systems which, in contrast to batch analysers, use the reagent continuously even when there is no sample present in the apparatus--notably during the start-up and shut-down procedures as well as through coffee breaks. This problem is not so great in FIA where the volume of the sample path is seldom larger than a few hundred microliters, and therefore easy to fill and wash in less than one minute with small amounts of reagent or wash solutions. If, however, an expensive reagent or an enzyme is used, it is wasteful to pump these solutions continuously, because the reagent occupies also these sections of the sample path where the sample zone momentarily is not present.

The merging zone principle avoids this expensive disadvantage by injecting the sample and introducing the reagent solutions in such a way that the sample zone meets the selected section of the reagent stream in a controlled manner, while the rest of the FIA system is filled with wash solution or only pure water. This can be achieved in two different ways, as shown in FIG. 6, by intermittent pumping (A) or by use of a multiple injection valve (B).

(A) Merging zone system with intermittent pumping

From top (a) the sample zone is injected into line I and propelled forward (by pump I) by an inert carrier solution, pump II administering the reagent solution being in the stop position. When the sample zone has reached the merging point (b), the pump II is activated delivering reagent, and pump I is stopped. After addition of reagent, pump I is reactivated and pump II stopped.

(B) The synchronous merging of two zones in a symmetrical system with continuous pumping

From top (a) equal sample and reagent columns are injected, (b) merge with identical velocities after passing through equal lengths of tubing, and (c) continue downstream while being mixed and dispersed into the carrier stream.

A merging zone system based on intermittent pumping is shown in FIG. 7. The length of the reagent zone can be regulated by simply selecting different start and stop intervals on the timer and creating different concentration gradients at the boundary surface between the sample zone, the reagent solution and the carrier flow.

FIG. 8 shows the record obtained with the FIA system in FIG. 7, demonstrating the merging zones principle performed with intermittent pumping. Pumping rate x=2.0 ml/min., y=z=1.0 ml/min. The shaded peak was recorded by injecting 20 μl of a bromthymol blue solution of concentration 1.0 into a colourless carrier stream, pumped by all three lines x, y and z.

Next, colorless solution was injected as well as pumped by lines x and y, while a bromthymol blue solution of concentration 0.5 was pumped through line z. While a delay time of 0.3 seconds was used in all experiments, pump II was activated for increasingly longer periods (a-5s, b-10s, c-15s, d-20s). The reagent zone broadens over the sample zone, until a "reagent steady state"--where D_(R) =1 is reached. Obviously 10 sec. pumping (curve b), corresponding to 166 μl of reagent per sample, is more than sufficient to cover the shaded sample zone.

The principle for merging zones by intermittent pumping is obviously to leave a time gap corresponding to a stop period between the actions of pump I and pump II so that the sample zone will be left either within the flow-through cell allowing the increase in the intensity of the colour formed to be measured (cf. FIG. 1, dotted line c), or outside the flow cell in a "waiting" position so that more colour is formed and the sensitivity of measurement thus increased. The first approach allowing the rate of chemical reactions to be measured is very important, for example in enzyme measurements, while the other case has proven to be valuable for certain trace analyses.

As an example of stop-flow rate-measurement with the zone within the flow cell, glucose in serum and SO₂ in wine can be mentioned and a very short mixing coil, e.g. 20 cm, is appropriate. The time sequence can be the same as that shown in FIG. 7.

Two or more reagents can also be added to a sample to measure two or more components in the sample which peak one after the other. This can be done, for example, by precipitating out two or more metals as hydroxides by adding a weakly acid or neutral sample containing the dissolved components, for example Cu, Pb and Cd, to a flowing liquor, e.g. 0.001 M NaOH and filtering out the precipitant by allowing pump I to run for a sufficiently long time to wash out the sample zone. Then pump II is started with the addition of a diluted acid which gradually dissolves the hydroxides. The plan for the apparatus is shown in FIG. 9.

Cd never precipitated, and the diluted acid first dissolves Pb and then Cu, so that the metal ions will pass through the filter and form a colour with the reagent to then be measured spectrophotometrically. Alternatively, a conductivity detector can be used if the reagent is a base exactly equimolar with the acid and the pumping rate y is exactly equal to z, because the change of conductivity will follow the amount of acid necessary to dissolve the hydroxide. This is a widely applicable principle. Finally, it should be pointed out that although this might seem to resemble chromatography, it is not; since the components to be separated form a solid second phase which is dissolved during each measuring cycle.

The merging zone concept might, if applied to a wider range of enzymatic assays, reverse the present trend towards the use of insolubilized enzymes, not only because of the high sampling rate and instant response of the FIA system, but also because rate measurements and blanking are not so readily accomplished in systems where enzymes are fixed in the form of columns or tubes.

The drawback of the merging zones approach is that a variable baseline signal will affect the measurement in an unfavourable way if the injected reagent zone alone is sensed by the flow-through detector. Thus in spectrophotometry the reagent should be colourless or at least should not absorb significant amounts at the wavelength at which the reaction product is being monitored. A very interesting aspect of the merging zones approach is the great versatility of this approach when performed by means of intermittent pumping. By choosing different lengths of reagent zone, one can obtain individual blanks for the reagent alone, the sample alone as well as the rate of formation of the reaction product. 

What we claim is:
 1. A method of analyzing a material within a flowing liquid carrier, comprising the steps of:(a) forming an entirely liquid flowing carrier stream; (b) injecting a discrete, well-defined liquid sample portion into said carrier; (c) introducing at least one reagent in the form of a liquid to said sample portion by intermittently activating separate pumps disposed to transport said carrier and said reagent respectively at pre-programmed timed intervals, activation of said pumps being controlled by the step of injecting said sample portion; and conducting the combination of said sample and said reagent to a flow-through detector for analysis while controlling dispersion of said sample in said conduit and said detector to optimize desired chemical reactions.
 2. The method of claim 1 wherein said method determines the rate of a chemical reaction, said method including the step of stopping the flow of said carrier when said sample is within said flow-through detector and measuring a change in said sample resulting from said reaction within said detector as a function of time.
 3. The method of claim 1 including the steps of: operating said pump disposed to transport said carrier at a rate significantly greater that the rate of the pump transporting said reagent; and activating said carrier pump subsequent to a maximum being registered by said detector.
 4. The method of claim 1 wherein several reagents are injected into said sample portion.
 5. The method of claim 1 including the step of stopping the flow of said carrier when said sample is within said flow-through detector.
 6. The method of claim 1 including the step of stopping the flow of said carrier before said sample enters said flow-through detector.
 7. The method of claims 1 or 6 including the step of operating said separate pumps to produce significantly different flow rates. 